Latest reports make the conclusion that an important tax gap is formed every year due to illegal, undeclared work. The gap’s size ranges from 300 to 480 million euros, representing tax and social-insurance revenues that the State of Finland does not receive. The results of the reports make it evident that Finland’s existing strategy for combating the shadow economy and the related action plan governing the cooperation between public authorities must be maintained. Undeclared work and the losses in fiscal revenue resulting from it were examined for the first time in Finland.